1. Technical Field
The present invention relates to an optical module and an atomic oscillator.
2. Related Art
In recent years, an atomic oscillator using coherent population trapping (CPT) as one of quantum interference effects has been proposed, and downsizing and reduction in power consumption of the device have been expected. The atomic oscillator using CPT is an oscillator using an electromagnetically induced transparency phenomenon (EIT phenomenon) that absorption of coherent light beams respectively having two wavelengths (frequencies) different from each other stops if an alkali metal atom is irradiated with the coherent light beams.
For example, in JP-A-2013-98606 (Document 1), there is disclosed an atomic oscillator in which the resonant light emitted from a light source is converted by a λ/4 plate into circularly polarized light, and the gas cell encapsulating the alkali metal atoms is irradiated with the circularly polarized light in order to increasing the occurrence probability of the EIT phenomenon. By irradiating the gas cell with the circularly polarized light, it is possible to cause an interaction between the alkali metal atoms and the circularly polarized light to thereby increase the probability of the alkali metal atoms existing in the ground level of magnetic quantum number mF=0. Thus, the occurrence probability of the EIT phenomenon can be increased.
The light generated in a surface emitting laser has a coherent property, and is therefore suitable for obtaining a quantum interference effect. In general, the surface emitting laser emits polarized light.
However, it is known that in the surface emitting laser, the polarization direction of the polarized light to be emitted changes, namely polarization switching occurs, due to an external factor (e.g., temperature, stress, aging of a device structure). In the single mode VCSEL used for the atomic oscillator, there are normally allowed two polarization directions perpendicular to each other. Therefore, in such a VCSEL, since the polarization direction changes from one of the two directions to the other, namely the polarization direction rotates 90 degrees, due to the polarization switching, it is not achievable to determine the polarization direction of the polarized light to be emitted to one direction.
Therefore, if the vertical cavity surface emitting laser (VCSEL) is applied as a light source of the atomic oscillator of Document 1, the polarization direction of the polarized light to be emitted from the vertical cavity surface emitting laser changes due to the polarization switching, and there is a problem that the polarization direction of the polarized light entering the λ/4 plate changes.